`files` option supported folder.

I have enabled the folder specified by the file option.

```javascript
files: {
    'tmp/folders/': ['test/fixtures/folders/']
}
```

result...

```
test/fixtures/folders/
└── jade.jade
tmp/folders/
└── jade.html
```

and customize file extension.

`extension: template` ...

```
files: {
    `tmp/folders/': ['test/fixtures/folders/']
},
options: {
    extension: 'template'
}
```

result...

```
test/fixtures/folders/
└── jade.jade
tmp/folders/
└── jade.template
```

Thanks.